HEIC0901: EMBARGOED UNTIL 12:00 (CET)/06:00 am EST 05 February, 2009
http://www.spacetelescope.org/news/html/heic0901.html
Photo release: Exceptionally deep view of strange galaxy
05-Feb 2009 A spectacular new image of an unusual spiral galaxy in the
Coma Galaxy Cluster has been created from data taken by the Advanced
Camera for Surveys on the NASA/ESA Hubble Space Telescope. It reveals
fine details of the galaxy, NGC 4921, as well as an extraordinary rich
background of more remote galaxies stretching back to the early
Universe.
The Coma Galaxy Cluster, in the northern constellation of Coma
Berenices, the hair of Queen Berenice, is one of the closest very rich
collections of galaxies in the nearby Universe. The cluster, also known
as Abell 1656, is about 320 million light-years from Earth and contains
more than 1000 members. The brightest galaxies, including NGC 4921
shown here, were discovered back in the late 18th century by William
Herschel.
The galaxies in rich clusters undergo many interactions and mergers
that tend to gradually turn gas-rich spirals into elliptical systems
without much active star formation. As a result there are far more
ellipticals and fewer spirals in the Coma Cluster than are found in
quieter corners of the Universe.
NGC 4921 is one of the rare spirals in Coma, and a rather unusual one —
it is an example of an “anaemic spiral” where the normal vigorous star
formation that creates a spiral galaxy’s familiar bright arms is much
less intense. As a result there is just a delicate swirl of dust in a
ring around the galaxy, accompanied by some bright young blue stars
that are clearly separated out by Hubble’s sharp vision. Much of the
pale spiral structure in the outer parts of the galaxy is unusually
smooth and gives the whole galaxy the ghostly look of a vast
translucent jellyfish.
The long exposure times and sharp vision of Hubble also allowed it to
not just image NGC 4921 in exquisite detail but also to see far beyond
into the distant Universe. All around, and even through the galaxy
itself, thousands of much more remote galaxies of all shapes, sizes and
colours are visible. Many have the spotty and ragged appearance of
galaxies at a time before the familiar division into spirals and
ellipticals had become established.
The Hubble images used to make this picture were originally obtained by
a team led by Kem Cook (Lawrence Livermore National Laboratory,
California). The team was using Hubble to search for Cepheid variable
stars in NGC 4921 that could be used to measure the distance to the
Coma Cluster and hence the expansion rate of the Universe.
Unfortunately the failure of the Advanced Camera for Surveys in early
2007 meant that they had insufficient data to complete their original
programme, although they hope to continue after the servicing mission.
Very deep imaging data like this, which is available to anyone from the
Hubble archives, may also be used for other interesting scientific
explorations of this galaxy and its surroundings
This image was created from 50 separate exposures through a yellow
filter and another 30 exposures through a near-infrared filter using
the Wide Field Channel of the Advanced Camera for Surveys on Hubble.
The total exposure times were approximately seventeen hours and ten
hours respectively.
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Notes for editors
The Hubble Space Telescope is a project of international cooperation
between ESA and NASA.
Image credit: NASA, ESA and K. Cook (Lawrence Livermore National
Laboratory, USA)
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